Devices for the separation of liquids from slurries are well known in the pulp and paper and mineral processing industries. One common device, the twin wire press, comprises a pair of endless mesh belts, guided over a series of rollers in a converging path. The slurry is introduced between the converging belts and compressive forces expel a large percentage of the liquid through the belts. An added benefit of removing liquid from the slurry is that dissolved solids and certain small particulates leave the slurry along with the liquid. The slurry is thereby rinsed, or washed, as the liquid is extracted. This process is known as dilution washing. High washing efficiencies may be achieved by using more liquid and/or by maximizing the outlet dryness of the dewatered slurry. Furthermore, if the dewatered slurry is rediluted with clean liquid and then rethickened, additional increases in washing efficiency can be achieved. This process is known as multi-stage dilution washing. Efforts are constantly being made to reduce the quantity of clean liquid required in order to achieve a given degree of washing efficiency.
Prior attempts to improve the washing process are disclosed in U.S. Pat. No. 2,111,720 to Padgett and U.S. Pat. No. 4,481,118 to Heissenberger et al.
The methods described by Padgett and Heissenberger et al have several disadvantages. Firstly they require a significant increase in the space requirements for the machinery required. Secondly the rinsing fluid cannot be applied in high volumes. Thirdly the rinsing fluid is applied to the outside of the pulp slurry and is subsequently removed from the outside of the pulp slurry. This limits the effectiveness of these methods. Fourthly it has been seen in practice that there is very little if any reduction in dilution liquid requirements when these methods are used.
The present invention overcomes the limitations of previous methods by introducing the rinsing fluid between two layers of pulp slurry thereby maximizing the rinsing effect of the liquid as it diffuses through the slurry. Accordingly only relatively small volumes of rinsing liquid are needed. A further advantage is that space requirements are not affected and there is no loss in drainage area of the mesh belts.